The present invention relates to a self-adhesive tape comprising pressure-sensitive adhesive layers laminated to one another.
Double-sidedly self-adhesive tapes are typically used for fixing and joining a wide variety of materials. The range of applications here is very diverse. Nowadays, for instance, a multiplicity of different self-adhesive tapes are used in the automotive industry, for example, for bonding door trim and decorative trim, and in the electronics industry, for example, for bonding displays, batteries or speakers in devices including mobile telephones, digital cameras or pocket computers. Through the use of pressure-sensitive adhesive tapes it is possible for the individual technical components to be mounted in a more space-saving way which is much quicker and hence more efficient and more advantageous in terms of cost.
For use in the automotive industry and electronics industry, the performance requirements imposed on the self-adhesive tapes are exacting. Important criteria here include a high bond strength, more particularly a high shear strength, high ageing resistance, and, not least, electronic compatibility. For these kinds of applications, therefore, self-adhesive tapes based on highly crosslinked polyacrylate adhesives are utilized primarily.
Selected applications impose particular requirements on thickness and thickness-tolerance on the part of the adhesive tapes. Thus, for example, for the bonding of display windows in mobile telephones, only self-adhesive tapes having a precisely selectable thickness and at the same time a very low thickness tolerance are suitable. The sum of the thickness of the display window and the thickness of the self-adhesive tape must fit exactly into the adhesive joint space left. A further example of a specific thickness requirement for self-adhesive tapes is in the manufacture of film keyboards. Here, self-adhesive tapes are typically used as spacers between the individual electrical functional layers. In this application, the use of adhesive tapes of 250 μm or even thicker is not uncommon. The applications necessitate a precisely defined thickness on the part of the self-adhesive tape. Setting the overall thickness of the self-adhesive tapes is typically done by adjusting the thickness of the carrier material—here, use is frequently made, for example, of polyester films—and/or by adjusting the thickness of the coating of adhesive. In the case of thick PET carrier films, the sharp increase in flexural rigidity that occurs as the thickness increases imposes restrictions on the maximum thickness that can be utilized. Typical commercial double-sided self-adhesive tape products therefore consist of a PET carrier film in a thickness, for example, of 9 to 175 μm, which is coated on both sides, for example, with 50 g/m2 of an acrylate adhesive. Products with even thicker PET carrier films, with thicknesses exceeding 200 μm, are very difficult indeed to process to form roll product, owing to the high flexural rigidity.
The second option, that of adapting the product thickness by way of the thickness of the coating of adhesive, is likewise subject to technical limits. In the processing of pressure-sensitive adhesives (PSAs) based on aqueous dispersions, maximum PSA thicknesses of typically around 70 g/m2 are achievable. Higher PSA thicknesses can be achieved if coating takes place from solvent. Subject to the proviso of a suitable drying technology, PSA thicknesses can be realised here of up to about 120 g/m2. One example of this that may be mentioned is the product Tesafix® 4965, which is composed of a PET carrier film 12 μm thick coated on both sides with 100 g/m2 per side of an acrylate adhesive. An even higher level of adhesive application is not realisable from an economic standpoint using the typical solvent processes, since bubble-free coating is impossible and/or drying is possible only with very slow operating speeds that are not feasible economically.
A further possibility for producing thick PSA coatings lies in the coating of PSA melts or of prepolymers which subsequently, in the as-coated state, are crosslinked. Disadvantages are the typically sharp limitation to the maximum molar mass that can be employed in the case of hotmelt PSAs, the often high residual levels of unreacted monomers, and oligomeric constituents when prepolymers are used.
A further possibility for the production of relatively thick PSA tapes is provided by the lamination of two or more pressure-sensitive adhesive (PSA) layers. This process operates very well in the case of non-crosslinked PSAs such as, for example, adhesives based on polyisobutylene, physically crosslinked PSAs, such as adhesives based on styrene block copolymers, for example, particularly when lamination here takes place at an elevated temperature, or PSAs with low levels of crosslinking, such as adhesives based on natural rubber with a low level of crosslinking, for example. The lamination of crosslinked acrylate PSA layers, in contrast, frequently leads to a laminate having a reduced profile of properties, owing to the low lamination strength or composite strength of the layers. It is thought that, owing to a high degree of crosslinking, the polymer chains of the acrylate PSA are incapable of forming loops of sufficient length at the interface. With certain products, this circumstance is utilized deliberately in order to offer a double-sided self-adhesive tape without release laminate release material (release paper or release film) in the form of roll product, so that two plies of the polyacrylate PSA layer come directly into permanent contact. Even after years of storage, these products can be unrolled without problems. Tesafix® 56661 is one example of such a product.